An Alien Dragnet

It seemed at first like A cumbersome bit of old-fashioned gumshoe work. Soon after the U.S. military went into Afghanistan, the FBI, the CIA and military intelligence began collecting fingerprints of al-Qaeda operatives and members of other international terrorist groups. They scoured caves and safe houses in Afghanistan and Pakistan and sought records from police and security services worldwide. Thousands of prints were digitized and entered into a classified database, along with names, aliases, mug shots, addresses, associates, descriptions of scars and, occasionally, DNA data.

Now, U.S. authorities say, this ambitious project is paying off. In the past three months, federal...